The invention applies generally to an apparatus for producing sound from light modulated at audio frequencies. More particularly, the invention relates to a photofluidic audio receiver which not only converts the light to sound but also amplifies the sound, and which delivers a flat frequency response over a preselected audio frequency range.
It is known in the prior art to use a photodiode to receive an optical signal and convert it to an electrical signal, which can then be amplified and used to power a conventional speaker or headset. However, this known scheme is sensitive to environmental hazards because the photodiode can become inoperative or be destroyed in the presence of electromagnetic radiation, extreme temperatures, or shock. Also, this scheme requires the use of electrical power, which can pose a safety hazard when the receiver is located in an environment, such as a coal mine, in which explosive gases may be present.
In a lead article entitled "The photophone--an optical telephone receiver", by D. A. Kleinman and D. F. Nelson, and two subsequent articles, published in the Journal of the Acoustical Society of America, Vol. 59, No. 6, June 1976, pages 1482-1494, and Vol. 60, No. 1, July 1976, pages 240-255, an optical telephone receiver employing the opto-acoustic effect is described. This receiver consists of an absorption cell, a response-equalizing device such as a gas column or diaphragm, a tapered acoustic tube acting as a transformer, and an earpiece similar to a conventional telephone earpiece including a response-equalizing device consisting of a diaphragm and screen. By use of these response-equalizing devices, this receiver gives a flat (3-dB) response to intensity modulated light over the telephone voice band 300-3300 Hz. This receiver is powered solely by the optical signal applied to it, and thus is suitable for use in a hazardous environment. However, its sound output will be limited by the power of the optical signal supplied to it, generally only a few milliwatts.
It is known in the prior art to use a laminar proportional amplifier (LPA) to amplify human speech. In a paper entitled "A Fluidic Audio Intercom" by T. M. Drzewiecki, 20th Anniversary of Fluidics Symposium, ASME, 1980, pages 89-94, a fluidic audio intercom suitable for use in a combat vehicle is described, in which a laminar proportional amplifier has an input connected to receive normal speech sound waves, and its outputs connected by air filled tubing to an airline head set. However, harmonic distortion and resonance in the air filled tubing limit the use of this system to distances of only a few meters.
In an article entitled "Photofluidic Interface" by J. O. Gurney, Jr., Journal of Dynamic Systems, Measurement, and Control, March 1984, Vol. 106, pages 90-97, and in U.S. Pat. No. 4,512,371, issued Apr. 23, 1985 to Drzewiecki et al., there is described a photofluidic interface for transducing optical control signals into fluid control pressures, in which a light source modulated at a predetermined frequency is utilized to transmit control signals to a photoacoustic cell that absorbs the light energy and converts it to heat energy to create pressure pulses within the cell. The output signal of the photoacoustic cell is then fluidically amplified by a laminar proportional amplifier, fluidically rectified by a fluidic rectifier, and again fluidically amplified by one or more LPA's to create a pneumatic or hydraulic output pressure which drives an actuator. The output pressure can be controlled by modulating the amplitude, pulse width, frequency, or gate width of the optical input signal.